This invention relates to a method for refining glyceride oils by contacting the oils with an adsorbent capable of removing certain impurities. More specifically, it has been found that amorphous silicas are quite effective in adsorbing both soaps and phospholipids from caustic treated or caustic refined glyceride oils, to produce oil products with substantially lowered concentrations of these impurities. For purposes of this specification, the term "impurities" refers to soaps and phospholipids. The phospholipids are associated with metal ions and together they will be referred to as "trace contaminants." The term "glyceride oils" as used herein is intended to encompass both vegetable and animal oils. The term is primarily intended to describe the so-called edible oils, i.e., oils derived from fruits or seeds of plants and used chiefly in foodstuffs, but it is understood that oils whose end use is as non-edibles are to be included as well. The invention is applicable to oils which have been subjected to caustic treatment, which is the refining step in which soaps are formed in the oil.
The terms "glyceride oil," "crude glyceride oil," "degummed oil," "caustic refined oil," "oil" and the like as used herein refer to the oil itself, including impurities and contaminants such as those discussed in this specification. These are substantially pure oils at about 99.8% or higher oil content (Handbook of Soy Oil Processing and Utilization, pp. 55-56 (1980)). This contrasts to solvent/oil solutions, or miscella as referred to by the industry. The initial oil extraction process in which oils are removed from seeds typically is done by solvent extraction (e.g., with hexane). The result is a solvent/oil solution which may be 70-75% solvent. Refining methods which utilize this solution commonly are referred to as miscella refining. This invention does not cover miscella refining. The glyceride oils utilized in the process described below are substantially pure oils, in the complete absence or substantially complete absence of solvents such as hexane. Thus, the method of this invention can be categorized as non-miscella refining.
Crude glyceride oils, particularly vegetable oils, are refined by a multi-stage process, the first step of which typically is "degumming" or "desliming" by treatment with water or with a chemical such as phosphoric acid, citric acid or acetic anhydride. This treatment removes some but not all gums and certain other contaminants. Some of the phosphorus content of the oil is removed with the gums. Either crude or degummed oil may be treated in a chemical, or caustic, refining process. The addition of an alkali solution, caustic soda for example, to a crude or degummed oil causes neutralization of free fatty acids to form soaps. This step in the refining process will be referred to herein as "caustic treatment" and oils treated in this manner will be referred to as "caustic treated oils. Soaps generated during caustic treatment are an impurity which must be removed from the oil because they have a detrimental effect on the flavor and stability of the finished oil. Moreover, the presence of soaps is harmful to the catalysts used in the oil hydrogenation process.
Current industrial practice is to first remove soaps by centrifugal separation (referred to as "primary centrifugation"). In this specification, oils which have been subjected to caustic treatment and primary centrifugation will be referred to as "primary centrifuged" oil. Conventionally, the primary centrifuged oil, which still has significant soap content, is subjected to a water wash, which dissolves the soaps from the oil phase into the aqueous phase. The two phases are separated by centrifugation, although complete separation of the phases is not possible, even under the best of conditions. The light phase discharge is water-washed oil which now has reduced soap content and may be referred to as "water-wash centrifuged" oil. The heavy phase is a dilute soapy water solution. Frequently, the water wash and centrifugation steps must be repeated in order to reduce the soap content of the oil below about 50 ppm. This fully water-wash centrifuged oil will be referred to herein as "caustic refined" oil. The water-washed oil then must be dried to remove residual moisture to below about 0.1 weight percent. The dried oil is then either transferred to the bleaching process or is shipped or stored as once-refined oil.
A significant part of the waste discharge from the caustic refining of vegetable oil results from the water wash process used to remove soaps. In fact, a primary reason for refiners' use of the physical refining process is to avoid the wastestream production associated with removal of soaps generated in the caustic refining process: since no caustic is used in physical refining, no soaps are generated. In addition, in the caustic refining process, some oil is lost in the water wash process. In the caustic refining process to which this invention relates, moreover, the dilute soapstock must be treated before disposal, typically with an inorganic acid such as sulfuric acid in a process termed acidulation. Sulfuric acid is frequently used. It can be seen that quite a number of separate unit operations make up the soap removal process, each of which results in some degree of oil loss. The removal and disposal of soaps and aqueous soapstock is one of the most considerable problems associated with the caustic refining of glyceride oils.
In addition to removal of soaps created in the caustic refining process, phosphorus-containing trace contaminants must be removed from the oil. The presence of these trace contaminants can lend off colors, odors and flavors to the finished oil product. These compounds are phospholipids, with which are associated ionic forms of the metals calcium, magnesium, iron and copper. For purposes of this invention, references to the removal or adsorption of phospholipids is intended also to refer to removal or adsorption of the associated metal ions. Adsorption of phosphorus on various adsorbents (for example, bleaching earth) has been practiced but only with respect to oils undergoing physical refining (in which no soaps are generated) or in caustic refining subsequent to water wash steps (in which the soaps are removed). No adsorption process has accomplished the removal of both soaps and phospholipids at an early stage of caustic refining where large quantities of soaps are present.